Most cinematographers start with a bachelor's degree in film and cinema. There's no way around it, really—they need to learn very precise information about a highly technical process. So get ready to study, because those cameras and lights can be complicated.
After schooling, an aspiring DP can usually land an apprenticeship or entry-level job to start building a résumé. Once there are a significant amount of credits on your résumé, you've learned your craft, and your past reels are impressive, you might be ready to strike out on your own.
Eventually, anyone who's serious about the career will go back to school to pursue a master's degree in cinematography. If you make it through that, you can start applying for some of the bigger and better work. There aren't many jobs to go around, so a DP needs to do everything within his or her power to get a leg up on the competition.